1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device of mounting a device chip by a flip chip bonding method and more particularly to a method of improving adhesion between a barrier metal used as an underlying metal film below solder balls and a surface protection film.
2. Description of Related Art
For further developing size-reduction of electronic equipments, it is an important factor how to improve the mounting density of parts. Also in semiconductor IC, a wireless bonding of directly connecting a bare chip of LSI to a conductor pattern on a mounting substrate has been proposed instead of an existent package mounting using bonding wires and a lead frame. Among all, a method of forming all electrode portions and bumps or beam leads to be connected therewith to a device chip on the device-forming surface and directly connecting them with the device-forming surface being downward to a conductor pattern on a mounting substrate is called as a flip chip bonding method, and generally utilized for the mounting of hybrid IC or application uses in large-scaled computers, since assembling steps can be made reasonable.
The flip chip bonding method includes several methods such as an Au beam lead method or solder ball (bump) method in each of which, an underlying metal film, so-called barrier metal, is formed between an Al electrode pad of IC and a bump material with an aim of improving the adhesion and preventing inter-diffusion. Particularly, in the solder ball method, since the barrier metal has an effect on a finished shape of the solder ball, it is referred to as BLM (Ball Limiting Metal).
For the constitution of the BLM film, a three-layered constitution comprising a Cr film, a Cu film and an Au film laminated in this order is most general. Among them, the Cr film at the lowermost layer functions as an adhesion layer to an electrode pad usually formed by using an Al system metal film, the Cu film as the intermediate layer functions as an anti-diffusion layer for the solder ball constituent metal and, further, the Au film at the uppermost layer functions as an antioxidant film for the Cu film, respectively.
Then, a process for forming a BLM film connected to an Al electrode pad by using a lift-off method is to be explained with reference to FIG. 15A to FIG. 15D. FIG. 15A shows a state of applying passivation to a substrate 61 and, further, resist patterning for defining a deposition range of the BLM film. Referring simply to the steps up to this state, an Al electrode pad 62 on a substrate 61 completed for the formation of all devices is patterned to a predetermined shape. Then, the entire surface of the substrate (wafer) is covered with an SiN passivation film 63, and an window is opened for exposing the Al electrode pad 62. The device chip is completed in this step.
Then, the entire surface of the wafer is covered with a first layer polyimide film 64 as a surface protection film, and the film is patterned to form an opening 64a for exposing the Al electrode pad 62. The BLM film is in contact with the Al electrode pad 62 by way of the opening 64a. Further, a photoresist coating film is formed for the entire surface of the substrate, and a resist pattern 65 is formed by way of photolithography and development. An opening 65a greater than the opening 64a is formed, being faced to the Al electrode pad 62 to the resist pattern 65.
Then, as shown in FIG. 15B, the resist pattern 65 is deformed into an overhang shape to form a deformed resist pattern 65d. The deformation is conducted by applying a reverse sputtering to the resist pattern 65 and thermally expanding the surface layer of the film.
Then, as shown in FIG. 15C, a Cr film, a Cu film and an Au film are successively formed by sputtering to deposit a BLM film. Since the flying direction of sputter particles is defined within a narrow range to the substrate surface in the sputtering method, the BLM film is not deposited on the side wall surface of the deformed resist pattern 65d having the overhang shape as described above. Accordingly, a BLM film 66a connected with the Al electrode pad 62 and a BLM film 66d deposited on the deformed resist pattern 65d are isolated and the latter BLM film 66b is an unrequited portion.
Further, when the wafer in this state is dipped in a resist peeling solution and put to a shaking treatment under heating, as shown in FIG. 15D, the deformed resist pattern 65 is peeled off and, at the same time, the unrequired BLM film 66b loses the deposition base and is removed while only the BLM film 66a connected to the Al electrode pad 62 is left.
Subsequently, a solder film for completely covering the BLM film 66a is formed, for example, by a lift-off method and, successively, heat reflow is applied. In this step, the solder film shrinks in self-alignment on the BLM film 66a by surface tension into solder balls. When the solder balls formed on the chip and the conductor pattern on the mounting substrate previously soldered are pressed while aligning and fused by heating, mounting of the chip is completed.
By the way, the Al electrode is usually disposed at the periphery of the device chip. However, as the device prepared into the chip is made finer and the distance with the Al electrode pad is reduced, it becomes difficult to form solder balls as usual. This is because contact between adjacent solder ball results in a worry of short-circuit.
However, if the diameter of the solder ball is decreased in order to avoid contact between the solder balls, the bonding strength between the mounting substrate and the device chip is lowered to deteriorate the reliability. Therefore, the present applicant has previously proposed a technique of changing the layout for solder balls while leaving the diameter thereof as usual and disposing the solder balls out of a region for forming the Al electrode pad (hereinafter referred to as rearrangement). In this technique, a wiring pattern up to a place for the rearrangement with the Al electrode pad is additionally required, and the wiring pattern is formed with the BLM film. Accordingly, since only the usual photomask pattern may be changed, the number of steps is not increased and this is extremely advantageous in view of the cost and the manufacturing efficiency.
FIG. 1 shows a portion of a device chip formed with solder balls. The lamination relation for each of material films constituting the device chip illustrated in the drawing is substantially identical with that shown in FIG. 15.
On the device chip, Al electrode pads 2a, 2b are arranged along a certain side. The Al electrode pads 2a, 2b are covered thereon with an SiN passivation film 3 having an opening 3a, and a first layer polyimide film 4 having an opening 4a to a further inside of the opening 3a successively, and connected with a BLM film 6 at the inside of the opening 4a. The BLM film includes two types. That is, they are BLM film 6a for a determined position patterned only just above the Al electrode pad 2a and a BLM film 6b for rearrangement extended to the outside of the region for forming the Al electrode pad 2b.
The entire surface of such a wafer is further covered with a second layer polyimide layers 7 shown by a dotted line in the figure, and openings 7a and 7b are formed in the second layer polyimide film 7. In this case, the opening 7a is formed just above the Al electrode pad 2a, while the opening 7b is formed to the outside of the region for forming a the Al electrode pad 2b. Solder balls 9ar, 9br are connected at the inside of the openings 7a, 7b to the Al electrode pads 2a, 2b by way of the BLM films 6a, 6b respectively, in which the former solder balls 9ar are formed at the positions identical with those in the existent case (determined position), while the latter solder balls 9br are formed at the positions different from the existent case (rearrangement). With such a layout, the solder balls are not brought into contact with each other upon fusion under heating.
However, in the actual process for rearrangement described above, there has come across cross a new problem for the insufficient adhesion of the BLM film 6b with the underlying portion. That is, in the existent process of forming the BLM film 6a only at the defined position, since a most portion at the bottom of the BLM film 6a is in contact with the Al electrode pad 2a, there is no problem for the adhesion with the underlying portion. However, as the BLM film 6b is extended for rearrangement, and the area of contact with the first layer polyimide film 4 is increased, the BLM film 6b is often peeled due to the insufficiency of adhesion between both of films. If such peeling should occur, strength for the solder bonding portions of assembled products by the flip*chip* bonding method can no more be ensured to give undesired effects on the reliability and the durability of the products.